General acid/base balance week 2

Question 1

State the normal [H+] and pH of plasma and ICF.

State the upper and lower limits of [H+] and pH that are compatible with life.

Answer 1

Range of [H+] compatible with life is about 20 to 160 nmol/L (pH 6.8 to 7.7).

• Plasma is normally about 40 nmol/L (pH 7.4)
• Intracellular fluid [H+] is about 100 nmol/L (pH 7.0)

Question 2

What are the 2 categories of acids? (just list)

Answer 2

1. carbonic acid (CA)
2. non-carbonic acids (HNC or NCA)

Question 3

What are some examples of non-carbonic acids?

What is the general equation for non-carbonic acids?

Answer 3

non-carbonic acids (HNC or NCA)

• sulfuric and phosphoric acids (ICF)
• lactic acid increases in hypoxemia
• ketones in diabetes and starvation
• notation: HNC ↔ H+ + NC

Question 4

Acid/base imbalance develops when sources (intake + production) and sinks (output + consumption) for acids (or bases) are not equal.

What processes/diseases/circumstances may lead to acid/base imbalance?

Answer 4

• hypo - or hyperventilation or abnormal pulmonary gas exchange changes the carbonic acid (CA) concentration because PCO2 changes.
• renal failure allows accumulation of non-carbonic acids (HNC) that are normally excreted in the urine.
• loss of fluids from the body that do not have the same acidity as the body.

Examples of fluid imbalances include:

• losses from the GI tract

o loss of acid gastric juice in vomiting

o loss of alkaline intestinal contents during diarrhea.

• metabolic changes cause accumulation of abnormal quantities of acidic metabolic intermediates
• lactic acid increases in hypoxemia
• ketones in diabetes and starvation

Question 5

What are the 4 classifications of acid/base imbalance?

For each, state the pH and the primary change that leads to the specific acid base disorder.

Answer 5

slide 45 of notes

Question 6

What are the 2 processes by which acid/base imbalances are limited or corrected? What organs are involved?

Answer 6

Change in acidity (or alkalinity) is limited or corrected by:

• buffering: chemical reactions which bind or release H+. A temporary expedient that prevents excessive changes in pH. Does not prevent pH from changing but does reduce the magnitude of the change that occurs. Is not a permanent “fix” because buffering “uses up” the buffer. To sustain the ability to buffer, the normal ratio of base to acid forms must be restored.
• compensation: changes in the rate of excretion of CO2 and/or the excretion or production of HCO3 – . Compensating organs are the lungs (which deal with CO2) and the kidneys (which deal with bicarbonate). Compensation is corrective. It returns the pH back to (or at least towards) normal and restores the base to acid ratio of buffer system to (or towards) normal.

Question 7

What makes up the carbonic acid buffering system? Where does this buffereing system primarily act?

List and explain 4 examples of non-carbonic acid buffering systems and where they act.

Answer 7

Carbonic acid buffering system (HCO3 – /H2CO3)

• Primary extracellular buffer system.
• Readily available in blood.

Non-carbonic acid buffering system

• hemoglobin (Hb– /HHb). Located in the RBC. Rapid circulation of the blood makes both Hb and blood HCO3- readily available to buffer at the GI, pulmonary, and renal interfaces (portals of material intake and loss).
• intracellular proteins and phosphates (Pr-/HPr; HPO4 2-/H2PO4-) are the most important buffers inside cells.
• plasma proteins are buffers. However, they are present in low molar concentration.
• bone CO3 2- is very slowly released during periods of acidosis.

Question 8

State the general time frames for the carbonic acid, Hb, cell proteins & phosphates, and bone carbonate buffering systems to complete their reactions.

Answer 8

Question 9

Suppose and individual is hypoventilating as a result of an overdose of a CNS depressant.

Describe the buffering reactions and compensatory reactions that would occur to restore pH back to normal.

Answer 9

Suppose an individual is hypoventilating as a result of an overdose of a CNS depressant.

Buffering reactions: PaCO2 would increase and the reversible reactions below would shift as shown (reaction 1).

↑CO2 + H2O → H2CO3 → H+ + HCO3-

HHb ← Hb- + H+

Much of the resulting H+ (reaction 1) reacts with Hb– (reaction 2). Hence, the change in [H+] is smaller than would have occurred without buffering by Hb. However, buffering leaves the body with less hemoglobin base, Hb– , to buffer H+ in the future. Note that CA is only buffered by NC buffers. The CA system cannot buffer itself.

Compensatory reactions: Later, the kidney will add new HCO3 – to the body, reversing reaction (1). H+ will then dissociate from HHb, reversing reaction (2).

CO2 + H2O ← H2CO3 ← H+ + ↑HCO3

HHb → Hb- + H+

This restores the pH, the lost Hb– , and the buffering ability of the Hb system.

Question 10

Explain the specific time frame for each type of buffering reaction that occurs in acid/base imbalance.

Explain the specific time frame for each type of compensatory reaction that occurs in acid/base imbalance.

Answer 10

Buffering reactions. NC acids are buffered by all of the body buffers. FIGURE 5 (attached) illustrates the time course of buffering and compensatory reactions.

• The added NC acid is most rapidly neutralized (i.e., buffered) by the plasma HCO3 – and plasma proteins. This requires a few minutes. FIGURE 5, curve 1
• Part of the remaining free H+ reacts with interstitial fluid HCO3 –. Equilibrium is reached in 15 min to ½ hour. FIGURE 5, curve 1
• H+ enters cells and is buffered by intracellular (IC) NC buffers. Hb– reacts faster than other IC buffers, requires ~1/2 to 1 hour. Others need up to 6 hours.FIGURE 5, curve 2

Compensatory reactions

• The CO2 generated by the buffering of H+ by HCO3 – is rapidly eliminated via the lungs. Arterial PCO2 changes very little because of the added acid.
• Respiratory compensation retains or blows off CO2. This is determined by the change in [H+]plasma acting on the peripheral chemoreceptors. PCO2 changes during compensation because of the nervous system resets the PCO2 level. The process is half complete in 6 hours; 95% in 12 hours. FIGURE 5, curve 3
• Renal (aka kidney) compensation occurs over 3 to 4 days. The kidneys increase H+ secretion in acidosis and decrease it in alkalosis. Thus, new bicarbonate is manufactured in acidosis and some filtered bicarbonate is not reabsorbed (excreted) in alkalosis.